Up to now, a vibration type angular velocity sensor using micro-electro-mechanical systems (MEMS) has been proposed. The vibration type angular velocity sensor employs a system in which a Coriolis force (also referred to as “deflecting force”; hereinafter, referred to as “Coriolis force”) generated when an angular velocity, which is an object to be detected, is input to the sensor from an outside, is detected as a displacement of vibration of a vibrator, while the vibrator is allowed to generate reference vibration with a predetermined amplitude. A direction of the Coriolis force is determined by a direction of the reference vibration and by a direction of the angular velocity. Accordingly, it is necessary that the vibrator have a structure in which the vibrator can easily generate vibration in the direction of the reference vibration and also in the direction of the Coriolis force (detecting direction). For this reason, there has been proposed a so-called double frame system in which a vibrator for reference vibration (reference vibration) and a vibrator for detection (detection vibrator) are separately provided.
Japanese Patent No. 3,336,730 discloses a structure, as one mode of the angular velocity sensor of the double frame system, in which the detection vibrator is supported by the reference vibrator, and a detection unit for detecting a relative displacement between the reference vibrator and the detection vibrator is provided. Accordingly, such a problem is alleviated that a component of the vibration of the reference vibrator which is generated in the detecting direction is superimposed with a detection signal of the detection unit.
Further, U.S. Pat. No. 6,374,672 discloses a structure in which an inclination of the detection vibrator corresponding to the angular velocity is detected by allowing a doughnut-type reference vibrator, which supports the disc-like detection vibrator, to generate reciprocating rotational vibration. As a result, such a problem is alleviated that degradation of the accuracy of the detection signal which is caused by the vibration of the detection vibrator due to the vibration of the reference vibrator.
In the vibration type angular velocity sensor, a displacement amount of the detection vibrator for detecting the angular velocity is generally only about one of many hundreds to one of many thousands of a displacement amount of the reference vibration of the reference vibrator. For this reason, it is necessary to detect a small motion of the detection vibrator in a large motion of the reference vibrator. Accordingly, when a noise component (e.g., vibration caused in a direction different from the direction of the reference vibration) of the vibration is generated by the reference vibration, the noise component is detected by the detection unit. In a case where the angular velocity detection is performed with high accuracy, the noise component may be a major factor of degradation of the accuracy in detection of the angular velocity.
In general, the reference vibrator has asymmetric properties caused by an error of a processing accuracy in a manufacturing process, with respect to a structure of the reference vibrator, a structure of a member for supporting the reference vibrator, a drive force applied by a drive unit of the reference vibrator, and the like. For this reason, the reference vibrator generates vibration with a wobbling component in a direction other than a predetermined direction of the reference vibration. The same is applied to the angular velocity sensor of the double frame system.
Causes of the noise generated by the reference vibration can be roughly classified into the following two causes. That is: (1) vibration itself of the reference vibration is detected as a detection signal in error by the detection unit; (2) vibration of the detection vibrator excited by the reference vibration (for example, also when an angular velocity ω is not input) is detected as a detection signal in error by the detection unit.
The above-mentioned causes are described with reference to the drawings with respect to the angular velocity sensor according to the above-mentioned cited references. In the angular velocity sensor disclosed in Japanese Patent No. 3,336,730, as illustrated in FIGS. 7B and 7C, a vibrating body 10 for reference and a vibrating body 16 for detection are separately provided, and the angular velocity is detected by a detection unit 15 based on a change of a relative position between the vibrating body 10 for reference and the vibrating body 16 for detection. Note that, FIG. 7C is a cross-sectional view taken along the line 7C-7C of FIG. 7B. In the structure, even when the vibrating body 10 for reference generates vibration in a direction (for example, an X-axis direction of FIG. 7B) different from a desired direction, the detection unit 15 detects the relative position between the vibrating body for reference and the vibrating body for detection. Accordingly, the noise is hardly generated in the detection signal. In other words, the noise due to the above-mentioned cause (1) can be reduced.
However, in order to detect the Coriolis force corresponding to the angular velocity, the vibrating body 16 for detection is supported by a structure in which the vibration toward the detecting direction (X-axis direction of FIG. 7B) is easily generated. Accordingly, when the reference vibration with a direction of linear reciprocating vibration in plane with the detecting direction has a component of the linear reciprocating vibration toward the detecting direction, small vibration (noise component of vibration) may be generated in the vibrating body for detection even in a case where the angular velocity is not input. As a result, in a case where the angular velocity is detected with high accuracy, the small noise generated in the detection signal by the above-mentioned cause (2) may degrade the detection accuracy.
The angular velocity sensor disclosed in U.S. Pat. No. 6,374,672, as illustrated in FIG. 7A, has a structure in which a reference vibrator 76 has a rotation axis perpendicular to a substrate 68, and a detection vibrator 70 has rotation axes 72 and 74 parallel to the substrate 68, thereby performing a reciprocating rotational motion about each of rotation axes as a center. In the structure, the reference vibrator and the detection vibrator perform the reciprocating rotational motion about the difference rotation axes. Accordingly, the vibration of the noise component of the detection vibrator 70 is hardly transmitted, thereby making it difficult to generate the vibration of the noise component of the detection vibrator 70. For this reason, as compared with the structure disclosed in Japanese Patent No. 3,336,730, an effect of the reference vibration on the detection vibrator can be reduced to a large extent. In other words, the noise due to the above-mentioned cause (2) can be reduced.
On the other hand, the detection unit detects an electrostatic capacity between a pair of semicircular detection electrodes (not shown), which are provided on a side of a substrate (bonded to the substrate 68) fixed to the both vibrators, and the detection vibrator 70 opposing to the pair of electrodes. By the semicircular detection electrodes, a positional relationship between the fixed substrate and the detection vibrator 70 can be detected.
However, in a case where the reference vibrator 76 generates reciprocating rotational vibration deviating from the rotation axis, or in a case where the reference vibrator 76 has a motion component with inclination with respect to the fixed substrate, even when the detection vibrator 70 is not oscillated, a relationship between opposing areas of the reference vibrator 76 and the pair of semicircular detection electrodes is changed. For this reason, by the above-mentioned cause (1), the small noise may be generated in the detection signal. Thus, when the angular velocity is detected with high accuracy, the small noise due to the above-mentioned cause (1) may degrade the detection accuracy.
In addition, In the angular velocity sensor disclosed in U.S. Pat. No. 6,374,672, the rotation axes 72 and 74 are deviated by the reference vibration of the reference vibrator 76. Specifically, in a case of detecting the inclination corresponding to the angular velocity with respect to the fixed substrate of the reference vibrator 70, the rotation axis of the detection vibrator 70 is deviated from a parallel line which separates the pair of semicircular detection electrodes provided on the fixed substrate side. Accordingly, to be exact, the wobbling component of the reference vibrator 76 may be mixed in the inclination component of the detection vibrator 70 which is detected by the detection unit. This may become a cause of degradation of the detection accuracy in the case where the angular velocity is detected with high accuracy (one reason for the above-mentioned cause (1)).